This invention relates to a method of trimming surface acoustic wave (SAW) devices and, more particularly, to trimming SAW resonant filters by dry etching either the metal electrodes or the substrate between the electrodes.
SAW devices find many applications in communications and other electronic equipment requiring filters having a very narrow frequency response, e.g., bandpass filters or oscillator frequency control elements. These SAW devices typically comprise a substrate on which is deposited one or more precisely formed and positioned interdigital transducers and, often, arrays of precisely positioned etched grooves. See, e.g., U.S. Pat. No. 4,144,507 entitled, "Surface Acoustic Wave Resonator Incorporating Transducer into Reflecting Arrays".
The application of these SAW devices has been accompanied by problems. These problems arise because any variation in groove depth, transducer metal thickness, or linewidth causes a shift in the center frequency and substantially alters the shape of the filter responses. In other words, a prior art surface wave acoustic device may display an improper center frequency and may also be unsuitable due to high insertion loss and/or the presence of a strong spurious mode.
To solve these problems, it is known in the prior art to "trim" the devices after fabrication. Several trimming techniques have been applied, but all have serious drawbacks. As an example, there is the dielectric overlay technique wherein trimming is achieved with a dielectric overlay, such as ZnO. However, using an overlay degrades both the circuit resonance Q and the long-term stability of the device. See, for example, W. R. Shreve, Proc. 1977 Ultrasonic Symposium, pp. 857-861.
Another example is the extra transducer technique. In this technique, electrical trimming is achieved by using an extra transducer inside the resonant cavity of the device. This technique, however, provides an insufficient trimming range on weakly coupling materials such as quartz. Furthermore, this technique increases the occurrence of spurious resonant modes in the device. See, for example, P. S. Cross, et al., Applied Phys. Lett., 28, 1-3 Jan. 1, 1976); C. Lardat, Proc. 1976 Ultrasonic Symposium, pp. 272-276.
Further, the laser technique is also used for trimming. The laser technique of trimming uses a laser to vaporize away sufficient material to alter the device characteristics. The technique, however, is restrictive. It is suitable only for trimming devices with metal reflectors, which inherently lower the Q of the devices. See, for example, W. R. Shreve, Proc. 1976 Ultrasonic Symposium, pp. 706-713.
Also used for trimming is the wet chemical etching technique. According to this technique, the device is trimmed by submerging the device in a bath of hydrochloric acid for wet chemical etching of the metal transducer material. However, this technique is usually insufficiently controllable for consistent results; the uncertainties in etch rates and the unavailability of direct monitoring of the frequency during the trimming process preclude efficient control in the technique.
Still another trimming technique is the groove masking technique. Trimming the SAW device by this technique is achieved by trimming the depth of the reflector grooves in the device, but leaving the substrate material between the electrodes untrimmed. See, for example, W. Tanski, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-26, No. 2, March 1979, pp. 93-104. This technique requires that some grating mask for the grooves, e.g., photoresist or metal, remain on the surface of the device during trimming. The subsequent removal of the mask, however, usually leads to an uncertain frequency shift in the device, thus nullifying the initial trimming process to a great extent. See, for example, R. C. M. Li, J. A. Alusow, and R. C. Williamson, Proc. 1975 Ultrasonic Symposium, pp. 279-283; C. A. Adams and J. A. Kusters, Proc. 31st Freq. Control Symposium, pp. 246-250 (1977).
Hence, there still exists a lack of a suitable method of fabricating SAW devices with a prescribed center frequency, a lack which hitherto has impeded the wider application of these devices.